I will add to the chorus here. I have used gmail to an iPod touch for the last 2 years and had no issue.
3 weeks ago I got an iphone4s and the push works erratically, and sometimes just not at all, I have to go in to mail.app and suddenly 20 emails appear.
I bought my mum an iPod touch for her birthday and it's the same. She had a MobileMe account which worked perfectly before on 2nd gen itouch, now on iOS5 it only works some days.
I wonder if anyone knows here if there is anywhere we can let apple know about this. Or do they already know? IT seems a lot of people are unable to get emails working in a reliable fashion - to me that's even more important than calls and I am for the first time in a long time looking enviously at android devices. I know I won't switch but it leave a bad taste in the mouth having just outlaid a lot of money and finding emails aren't working properly - I have been through every setting, into the advanced menu etc - no matter what, every few days it goes wrong. This applies to gmail, mobileme-to-icloud transitioned accounts, iPod touches and iphone4s. I'm in the UK if that makes any difference. Probably my worst experience with apple software in 6 years.